Polystyrene (PS) is one of the largest volume thermoplastic resins in commercial production today. This ubiquitous material is well suited to many "low performance" applications wherein its brittle nature is of little consequence. Additionally, many other applications requiring greater impact resistance have been uncovered by the advent of various modifications of these plastics. Thus, styrene-based copolymers, and particularly PS resins which are modified with organic rubber particles, have been a commercially viable alternative to some of the more exotic and expensive engineering plastics for certain applications.
One such system, known in the art as high impact polystyrene (HIPS), can have an impact strength which is an order of magnitude greater than the virgin resin but suffers from poor thermal stability, particularly in the presence of oxygen. These modified PS resins are typically prepared by polymerizing a solution of an unsaturated organic rubber, such as polybutadiene, in styrene monomer.
The addition of various rubber compositions to other thermoplastic resin systems has also proved beneficial. For example, Japanese Kokai Patent Application No. Hei 2(1990)-263861 to Mitsubishi Rayon Co., Ltd. discloses a thermoplastic resin composition having a high impact strength, high heat resistance and good resistance to organic solvents. This composition comprises a blend of polyphenylene ether (PPE) resin, a polyester resin and a rubber-like elastomer and/or modified rubber-like elastomer. A preferred elastomer component of this prior art disclosure is obtained by the graft copolymerization of at least one vinyl monomer with a composite rubber consisting of a silicone rubber and a polyalkyl methacrylate "interlocked with each other in an inseparable way." In the production of the composite rubber component, a cyclic diorganosiloxane is emulsion polymerized with a crosslinker and, optionally, with a graft crosslinking agent using a sulfonic-acid-series emulsificating agent. In a subsequent step, a combination of an alkyl (meth) acrylate, a crosslinker and a graft crosslinking agent is used to swell the silicone particles of the above emulsion and an initiator is then introduced to polymerize this system.
In a similar approach, U.S. Pat. No. 5,047,472 to Alsamarraie et al. teaches thermoplastic molding compositions comprising PPE resin, or a PPE resin containing a polystyrene resin, which is modified with a multi-stage polyorganosiloxane/vinyl-based graft polymer. These compositions are stated to have improved impact resistance, flame resistance and moldability. In this case, the graft copolymer is prepared by a "co-homopolymerization" technique wherein an emulsion containing a diorganosiloxane, a crosslinker and a graft-linker is polymerized concurrently with the polymerization of a vinyl monomer. The resulting first stage polymeric co-homopolymerized substrate is then grafted with a vinyl polymer in at least one subsequent stage. This multi-stage polydiorganosiloxane polyorganosiloxane/vinyl-based graft polymer formed according to the methods described by Alsamarraie et al. was also employed by Derudder et al. in U.S. Pat. No. 4,939,205 to augment the impact resistance of polycarbonate resin compositions. The graft polymer was further used by Wang in U.S. Pat. No. 4,939,206 to modify various thermoplastic resins with the object of providing flame retardant compositions having improved impact resistance.
U.S. Pat. No. 4,690,986 to Sasaki et al. teaches a thermoplastic resin composition having improved impact strength, achieved by combining a silicone rubber powder with an organic monomer. The silicone rubber powder is obtained by an emulsion co-polymerization of a polydiorganosiloxane oligomer, a tetra functional silane crosslinker and a grafting silane. However, the method of the present invention also differs from the method taught by Sasaki et al. at in least three key respects. First, Sasaki et al. and the comparative experiment include a tetrafunctional crosslinking agent, whereas this component is missing in the method of the application. Second, the 3-methacryloxypropyltrimethoxysilane, polydimethylsiloxane and tetrafunctional crosslinker are mixed and copolymerized in the emulsion of the comparative example and in the method of Sasaki et al. These latter methods lead to a random distribution of crosslinks across the backbone of the emulsion copolymer, because either the grafting silane or the tetrafunctional crosslinker can attach at any point along the backbone of the siloxane molecule. In the method of the present invention, an emulsion polymer having a molecular weight greater than 50,000 is first formed and only then crosslinked in a condensation reaction with a trifunctional silane. Moreover, the condensation reaction of the present invention takes place in the presence of a tin catalyst, which is not disclosed in the Sasaki et al. The condensation reaction of the present invention results in a controlled distribution of crosslinks since crosslinking takes place only at the polymer chain ends. The average molecular weight between crosslinks would therefore, be expected to be greater than that of the corresponding emulsions of the comparative experiment and those of Sasaki et al.